US20040227932A1 - Large dynamic range shack-hartmann wavefront sensor - Google Patents
Large dynamic range shack-hartmann wavefront sensor Download PDFInfo
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- US20040227932A1 US20040227932A1 US10/778,888 US77888804A US2004227932A1 US 20040227932 A1 US20040227932 A1 US 20040227932A1 US 77888804 A US77888804 A US 77888804A US 2004227932 A1 US2004227932 A1 US 2004227932A1
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J9/00—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength
Abstract
A wavefront sensor for measuring a wavefront contains an array of lenslets, a detector array, and a mask having a temporally fixed pattern containing one or more opaque regions that are substantially opaque to light from the wavefront. The mask comprises one or more transmissive regions that are transmissive of light from the wavefront. The mask and the array of lenslets are disposed such that light from the wavefront that is transmitted by the transmissive regions is focused by onto the detector array by the array of lenslets. The mask is adapted to be selectably disposed to any one of a plurality of predetermined positions, wherein a different group of lenslets from the array focuses light from the wavefront onto the detector array depending on which of the plurality of predetermined positions is selected.
Description
- This application claims the priority benefit of U.S. Provisional Application No. 60/447,344, filed Feb. 13, 2003, the entirety of which is hereby incorporated by reference.
- 1. Field of the Invention
- The present invention is directed to a wavefront measuring device, and more specifically, to a Shack-Hartmann type wavefront sensor with a large dynamic range.
- 2. Description of the Related Art
- The Shack-Hartmann technique is commonly used for determining wavefront shape or error from an ideal planar wavefront. The Shack-Hartmann wavefront sensor is a slope measurement device typically comprising a lenslet array, a two-dimensional detector array, acquisition hardware, and analysis software. Each lenslet in the array receives light from a portion of an incident wavefront. Light from the lenslet is focused within a “virtual” subaperture of the detector array, the detector subaperture generally being defined by those pixels disposed within a projection of the lenslet onto the detector array. The location of the focused light from a particular lenslet within each of these detector subapertures is used to determine the nominal slope of that portion of the incident wavefront. By calculating the slope of the incident wavefront from each spot displacement at each of the lenslets, the shape of the wavefront can be determined.
- The dynamic range of a Shack-Hartmann wavefront sensor is typically based on the focal length of the lenslets and the dimensions of the detector subaperture, in units of pixel number, for each lenslet. In prior-art systems, the combination of lenslet focal length and detector subaperture dimensions usually limits the maximum wavefront slope that can be measured. If the slope of a wavefront at one or more of the lenslets exceeds such a predetermined limit, the focus spots from such lenslets move into the subaperture of another lenslet, resulting in one of the following problems: (1) multiple spots are created within a single subaperture, (2) multiple spots overlap within a single subaperture, and (3) spots switching between subapertures. For instance, if the wavefront slope in the area of a first lenslet in the array exceeds this maximum, the light received by the first lenslet produces a focus that is outside the bounds of a corresponding first detector subaperture and is instead received by in a second detector subaperture corresponding to a second lenslet in the array. The presence of the focus from the first lenslet in the second detector subaperture results in an ambiguity, since it cannot be determined, a priori, from which lenslet the focused light came.
- Which of the three listed problems is produced depends on what happens with the focus spot from the second lenslet. If the wavefront slope at the second lenslet does not exceed the maximum limit, problems (1) or (2) can result. In the case of problem (1), it is indeterminate which spot belongs to which lenslet. In the case of problem (2), the focus of the second lenslet is indeterminate, since there is insufficient information to determine whether the second focus spot is located at that of another lenslet or the second focus spot is absent. If the wavefront slope at the second lenslet does exceed the maximum limit, problem (3) results. In this case an error can results since the focus spots will usually not be associated with the correct lenslet. These problems can exist between two lenslets or several lenslets.
- One solution to increase the dynamic range is to decrease the focal length of lenslets in the lenslet array. The result of such a design choice is to increase the amount of wavefront slope needed to exceed the bounds of the corresponding detector subaperture. The drawback to this choice is that the sensitivity of the wavefront sensor is decreased proportionately if all other system parameters remain the same as they were in the longer focal length lenslet design.
- Another method of increasing the dynamic range is suggested in an article by Lindlein, et. al. (“Algorithm for expanding the dynamic range of a Shack-Hartmann sensor by using a spatial light modulator array”, Optical Engineering, 40(5) 837-840 (May 2001)). Lindlein et. al. disclose the use of a spatial light modulator (SLM) to create a sequence of switching patterns that mask differing sets of lenslets in the lenslet array of a Shack-Hartmann sensor. Use of the switching patterns removes the requirement that each lenslet focus light within a detector subaperture. Using the method disclosed by Lindlein et. al., the focus spots formed by light from each lenslet may be located anywhere on the detector, with the exception that “spots are not allowed to overlap”. The authors calculate the minimum number of switching patterns necessary to provide an unambiguous correlation between wavefront slopes and the focus spot locations on a sensor array.
- The authors also provide an algorithm for determining which lenslet array subapertures are “switched off” in each switching pattern. For instance, an array of 40 lenslets by 40 lenslets would require nine different switching patterns. Each switching pattern has a form that is different from the other. The Lindlein et. al. method preclude taking a fixed switching pattern and simply moving the pattern to a different coordinate at each step in the sequence.
- A need exist, therefore, for providing a simple device and method for resolving ambiguities produced in Shack-Hartmann type wavefront sensor that are created by large wavefront slopes, thus increasing the dynamic range of such wavefront sensors.
- One way of increasing the dynamic range of a Shack-Hartmann wavefront sensor is by blocking and unblocking individual lenslets within the array thereof in a temporally predetermined manner. While a particular lenslet is blocked, the detector subaperture associated with that lenslet is precluded from receiving light incident on that lenslet. Thus, the detector subaperture for the blocked lenslet is available to receive a signal from another, unblocked lenslet in a potentially unambiguous manner. The blocked lenslet may then be unblocked while simultaneously blocking other lenslets in a prescribed manner. Thus, a predetermined sequence of blocking lenslets within the lenslet array may be used to increase the dynamic range of a Shack-Hartmann wavefront sensor.
- One aspect of the present invention involves a device for measuring a wavefront. The device comprises an array of lenslets, a detector,array, and a mask having a temporally fixed pattern containing one or more opaque regions that are substantially opaque to light from the wavefront and one or more transmissive regions that are transmissive of light from the wavefront. The mask and the array of lenslets are disposed such that light from the wavefront that is transmitted by the transmissive regions is focused onto the detector array by the array of lenslets. The mask is adapted to be selectably disposed to any one of a plurality of predetermined positions, wherein a different group of lenslets from the array of lenslets focuses light from the wavefront onto the detector array depending on which of the plurality of predetermined positions is selected.
- In yet another aspect of the present invention a method for measuring a wavefront comprises providing a wavefront sensor containing a detector array, an array of lenslets, and a mask having a temporally fixed pattern containing one or more opaque regions that are substantially opaque to light from the wavefront and one or more transmissive regions that are transmissive of light from the wavefront. The method further comprises disposing the array of lenslets such that two lenslets from the array of lenslets are capable of focusing light from the wavefront onto a point on the detector array. The method additionally comprises disposing the mask such that only one of the two lenslets focuses light from the wavefront onto the point.
- Another aspect of the present invention involves a method for measuring a wavefront comprises providing a wavefront sensor containing a detector array, an array of lenslets, and a mask having a temporally fixed pattern containing one or more opaque regions that are substantially opaque to light from the wavefront and one or more transmissive regions that are transmissive of light from the wavefront. The method also comprises disposing the mask to a first location wherein a first plurality of lenslets from the array of lenslets focuses light from the wavefront onto the detector array. The method further comprises moving the mask to a second location wherein a second plurality of lenslets from the array of lenslets focus light from the wavefront onto the detector array.
- Yet another aspect of the present invention involves a device for measuring a wavefront containing a detector array and a spatial light modulator (SLM) having a first plurality of zones and a second plurality of zones. The first plurality of zones is adapted to substantially block light from a first portion of the wavefront such that light from the first portion of the wavefront is not received by the detector array. The second plurality of zones is adapted to form a plurality of focusing elements that focus light form the wavefront to produce a corresponding plurality of foci on the detector array. The plurality of foci produces a plurality of signals for estimating the slope of the wavefront at the plurality of focusing elements.
- Still another aspect of the present invention involves a method for measuring a wavefront comprises providing a wavefront sensor containing a detector array, a lens, and a mask having an aperture adapted to transmit from light from the wavefront. The method additionally comprises disposing the mask to a first location, wherein light from a first portion of the wavefront is transmitted by the aperture and is focused by the lens onto the detector array to produce a first signal. The method further comprises moving the mask to a second location, wherein light from a second portion of the wavefront is transmitted by the aperture and is focused by the lens onto the detector array to produce a second signal. The method also comprises using the first signal to determine the slope of the first portion of the wavefront and using the second signal to determine the slope of the second portion of the wavefront.
- The foregoing features, aspects, and advantages of the present invention will now be described with reference to the drawings of preferred embodiments that are intended to illustrate and not to limit the invention. The drawings comprise ten figures.
- FIG. 1 is a side view of a wavefront sensor for measuring a wavefront according to embodiments of the present invention.
- FIG. 2 is a front view of an mask of lenslets used in certain embodiments of a wavefront sensor for measuring a wavefront.
- FIG. 3 is a front view of a array used in certain embodiments of a wavefront sensor for measuring a wavefront.
- FIG. 4 is a schematic illustration showing a magnified side view of a lenslet and a portion of a detector array for a prior-art Shack-Hartmann wavefront sensor
- FIG. 5a is a side view of a prior-art Shack-Hartmann wavefront sensor.
- FIG. 5b is a side view of a prior-art Shack-Hartmann wavefront sensor having a larger dynamic range than the wavefront sensor shown in FIG. 4.
- FIG. 6 is a side view of wavefront sensor according to an embodiment of the present invention.
- FIG. 7 is a front view of mask overlaying a lenslet array as the mask is moved to different locations in accordance with an embodiment of the present invention.
- FIG. 8 is a side view of wavefront sensor according to another embodiment of the present invention.
- FIG. 9 is a side view of wavefront sensor comprising a single lens and a mask having a single aperture
- FIG. 10 is a front view of a spatial light modulator having regions that form lenslets that focus light and other regions that block light.
- These and other embodiments of the present invention will also become readily apparent to those skilled in the art from the following detailed description of preferred embodiments having reference to the attached figures; however, the invention is not limited to any particular embodiment(s) disclosed herein. Accordingly, the scope of the present invention is intended to be defined only by reference to the appended claims.
- Wavefront Sensor
- FIGS. 1, 2, and3 schematically illustrate a
wavefront sensor 10 for measuring awavefront 15. Thewavefront sensor 10 comprises anarray 20 oflenslets 25, adetector array 30, and amask 35 having a temporally fixedpattern 40 containing one or moreopaque regions 45 that are substantially opaque to light from thewavefront 15 and one or moretransmissive regions 50 that are transmissive of light from thewavefront 15. Themask 35 and thearray 20 oflenslets 25 are disposed such that light from thewavefront 15 that is transmitted by thetransmissive regions 50 is focused by onto thedetector array 30 by thearray 20 oflenslets 25. Themask 35 is adapted to be selectably disposed to any one of a plurality of predetermined positions, wherein a different group oflenslets 25 from thearray 20 focuses light from thewavefront 15 onto thedetector array 30 depending on which of the plurality of predetermined positions is selected. The light from thewavefront 15 that is focused on thedetector array 30 forms a plurality of focus points 55. The locations of the plurality of focus points 55 may be correlated to the nominal slope of thewavefront 15 over the aperture of each lenslet 25 focusing light from thewavefront 15. - The
array 20 oflenslets 25 is preferably disposed in a two-dimensional grid that samples at least a portion of thewavefront 15. For example, FIG. 3 schematically illustrates an embodiment wherein thearray 20 oflenslets 25 comprises a grid pattern having 5 rows by 5 columns oflenslets 25. Alternatively, other patterns may be advantageously used, such as a hexagonal pattern. Thearray 20 may optionally be disposed to form a single row or a single column oflenslets 25. Preferably, thearray 20 oflenslets 25 has a fill factor that approaches to one; however, this is not critical to the operation of thewavefront sensor 10, which may, in principal, be used when thearray 20 oflenslets 25 has a fill factor that is much less than one. For example, for thearray 20 oflenslets 25 illustrated in FIG. 3, eachlenslet 25 has a circular cross-section when viewed from the front. In such cases, the fill factor is approximately 0.785 (π/4). Alternatively, eachlenslet 25 may have a cross-section that is substantially square or rectangular when viewed from in front of thearray 20 oflenslets 25. In such cases, the fill factor is approximately one. Other cross-section may also be used consistent with embodiments of thewavefront sensor 10. - When disposed in the form of a two-dimensional grid, the lenslets have a nominal spacing along the horizontal and vertical axes of the figure of sx and sy, respectively. Preferably, the magnitudes of the spacings sx, sy are substantially equal, wherein the nominal spacing is designated as s (=sx=sy);however, unequal values of the magnitudes of the spacings sx and sy are also consistent with embodiments of the present invention. The diameter of the
lenslets 25 along the horizontal and vertical axes is preferably substantially equal to the magnitudes of the spacings sx, sy. The diameters of thelenslets 25 along the horizontal and vertical axes is preferably small enough so that only a small portion ofwavefront 15 to be sampled by eachlenslet 25. Eachlenslet 25 has a diameter that is preferably between about 100 micrometers and 2 millimeters; however, lenslet diameters above or below this range are compatible with embodiments of the invention. - Ordinarily, the
array 20 is substantially square and has an equal number oflenslets 25 along the horizontal and vertical axes; however, there is no requirement that either of these conditions be true. For example, if there are more horizontal pixels than vertical pixels for aparticular sensor array 30, it may be it desirable to use aarray 20 oflenslets 25 that has more horizontal lenslets than vertical lenslets. - In certain embodiments, the wavefront sensor is used to measure a
wavefront 15 originating from a human eye. In such embodiments, thearray 20 oflenslets 25 is square or rectangular and has horizontal and vertical diameters that are preferably at least about 8 millimeters. In other applications of thewavefront sensor 10, the size and shape of thearray 20 may be otherwise configured to conform to predetermined design parameters of the system or wavefront being measured. The number of lenslets along each of the horizontal and vertical axes of thearray 20 will depend on the size of thewavefront 15 being measured, the size and focal length of thelenslets 25, and the desired wavefront slope resolution. Generally, the number of lenslets along each of the horizontal and vertical axes of thearray 20 preferably in a range of approximately 4 to 80 lenslets. For a givensize detector array 30, those skilled in the art can determine the optimum number of lenslets appropriate for a set of design constraints. For instance, as the number of lenslets increases the wavefront slope is measured at more locations over thewavefront 15; however, for a givendetector array 30, the number pixels within a subaperture is reduced. This may result in a decrease in the resolution or dynamic range of the wavefront slope measurement. It is envisioned that as the state of the art for the fabrication of lenslet and sensor arrays advances, even larger numbers of lenslets will become both possible and desirable. - In certain embodiments, each of the
lenslets 25 focuses light from thewavefront 15 by using refraction. In such embodiments, eachlenslet 25 has afront surface 60 and back surface 65 that may be spherical in shape and made of a commonly used optical material such as fused silica or silicon. Alternatively, either or both of thesurfaces array 20 oflenslets 25 comprises a diffractive optical element that focuses light from thewavefront 15 based on diffractive interaction with each lenslet. - In certain embodiments, the
lenslets 25 each have a nominal focal length of f and a nominal diameter d that is substantially equal to the spacing s of thelenslets 25. Each of thelenslets 25 also has anoptical axis 70 defined by a line passing through the center of thelenslet 25 and extending in a direction that is approximately normal to the center portion of theback surface 65 of eachlenslet 25. - Various fabrication techniques are common in the art for producing the micro-lenses from which the
array 20 oflenslets 25 is comprised. Such techniques include molding technology, ink-jet printing technology, and photolithography. Such techniques may be used produces lenslets 25 are either refractive or diffractive in nature. For instance, one manufacturer uses a photolithographic process that includes designing a gray-scale mask that is used to pattern a photoresist-coated substrate. The gray-scale mask has a high-resolution pattern with a range of optical densities that are used in the photolithographic process to pattern the photoresist. This pattern is then etched into the substrate using a plasma-etch process. Using such processing, the manufacturer can fabricate a lenslet with virtually any desired shape. - The
detector array 30 is preferably a one or two dimensional sensor array such as a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) detector array. In certain embodiments, thedetector array 30 produces a signal comprising the locations of the plurality of focus points 55 and a computer or similar such device receives the signal for processing information contained in the signal. As used herein, the term “focus point” is a broad term and is used in its ordinary sense and refers, without limitation, to the small area defined by the intersection of light from a focused wavefront with a plane disposed normal to the optic axis of the focusing element and near the circle of least confusion characteristic of such focused wavefronts. - The
detector array 30 may additionally comprise a plurality ofdetector subapertures 80, each detector subaperture 80 corresponding to alenslet 25 in thearray 20. In certain embodiments, thedetector subapertures 80 represent a grouping of pixels fromdetector array 30 rather than a physical boundary. Each detector subaperture 80 generally comprises those pixel of thedetector array 30 located within the projection of the correspondinglenslet 25 from thearray 20. Preferably, the direction of such a projection from the correspondinglenslet 25 is along theoptical axis 70 of the correspondinglenslet 25. - In certain embodiments, the
mask 35 comprises a substantially flat substrate such as a plate, film, or sheet havingopaque regions 45 andtransmissive regions 50. Thetransmissive regions 50 themask 35 may comprise areas where material is partially or completely removed from themask 35. Alternatively, thetransmissive regions 50 may comprise a substance or material that transmits at least a portion of light in the waveband of thewavefront 15. Theopaque regions 45 preferably comprise a substance or material does not transmit any light in the waveband of thewavefront 15. In certain embodiments, theopaque regions 45 are partially transmissive of light in the waveband of thewavefront 15, but in any event, the amount of light transmitted by theopaque regions 45 is less than the amount of light transmitted by thetransmissive regions 50. In other embodiments, theopaque regions 45 transmit light in the waveband of thewavefront 15, but that light is at least partially diffused such that thelenslets 25 corresponding to theopaque regions 45 do not produce focus points 55. Alternatively, in such embodiments, thelenslets 25 corresponding to theopaque regions 45 produce focus points that are sufficiently weak in intensity so as to be distinguished from the focus points 55 corresponding to the transmissive regions. - In certain embodiments, the
mask 35 comprises a substrate material that is at least partially transparent to light in thewavefront 15 such as silicon, fused silica, or plastic material. Theopaque regions 45 of themask 35 may comprise a material that is deposited material that is substantially opaque to light in thewavefront 15. For instance a material such as silver or aluminum may be applied to theopaque regions 45 using techniques such as vapor deposition or lithography. In other embodiments, a paint, ink, or other suitable pigment may be applied to one of both sides of themask 35 to provide theopaque regions 45. - In yet other embodiments, the
mask 35 comprises a substrate material that is substantially non-transmissive of light in thewavefront 15 such as a plastic material. In such embodiments, thetransmissive regions 50 of themask 35 may be formed by physically removing some of the substrate material from those regions. Alternatively, the optical properties of substrate material in thetransmissive regions 50 may be altered chemically so that those regions of themask 35 are more transmissive of light in thewavefront 15. - In still other embodiments, the polarization characteristics of the
mask 35 are varied such that theopaque regions 45 and thetransmissive regions 50 appropriately block and transmit polarized light from thewavefront 15. Alternatively, thetransmissive regions 50 of themask 35 do not directly transmit light from thewavefront 15, but comprise a material, such as a fluorescent dye, that absorbs energy from thewavefront 15 and remits light that is directed to thedetector array 30. - In other embodiments, the
mask 35 comprises a spatial light modulator (SLM) or similar such device havingopaque regions 45 andtransmissive regions 50. In such embodiments, theopaque regions 45 are defined as those regions of the SLM in which light from thewavefront 15 passing through the SLM changes polarization by an amount sufficient to substantially preclude transmission through a polarizer located at the output of the SLM. In such embodiments, thetransmissive regions 50 are defined as those regions of the SLM in which light from thewavefront 15 passing through the SLM changes polarization by an amount sufficient to be at least partially transmitted by through the polarizer located at the output of the SLM. The SLM may comprise a liquid crystal display (LCD), an array of addressable micro-mirrors, or another similarly such pixelated device that addressably varies one or more optical properties (e.g., polarization, phase, attenuation) over the surface of an incident wavefront. - In certain embodiments, the
pattern 40 of themask 35 is temporally fixed. The term “temporally fixed” as used herein and applied to thepattern 40 refers, without limitation, to a pattern in which the overall shape and size of the pattern and the components thereof (e.g., theopaque regions 45 and thetransmissive regions 50 of the mask 35) do not substantially change over time. In certain embodiments, as discussed in greater detail herein below, thepattern 40 of themask 35 is temporally fixed and spatially variable. The terms “spatially variable” and “varied spatially” as used herein and applied to thepattern 40 refers, without limitation, to a pattern that changes position over time, while the overall shape and size of the pattern and the components thereof remain substantially constant. - The apertures created on the
mask 35 by thetransmissive regions 50 preferably have substantially the same area and shape as thelenslets 25 when view from the front. Alternatively, each of thetransmissive regions 50 may have an area and extent that is smaller than theindividual lenslets 25 in thearray 20, such as shown for the two-dimensional mask in FIG. 2. In some embodiments, thetransmissive regions 50 have a size, shape, and extent consistent with certain performance and/or fabrication constraints. - As illustrated in FIG. 1, the
mask 35 may be disposed such that thearray 20 oflenslets 25 is between themask 35 and thedetector array 30. In such configurations, it is preferred, but not required, that thetransmissive regions 50 do not transmit any light in the waveband of thewavefront 15. Alternatively, themask 35 may be disposed such that themask 35 is between thearray 20 oflenslets 25 and thedetector array 30. - Shack-Hartmann Wavefront Sensor
- FIG. 4 is a schematic illustration showing a magnified side view of a lenslet25 a and a portion of the
detector array 30 for a prior-art Shack-Hartmann wavefront sensor illustrating how the lenslets 25 a focuses light from a portion 75 a of thewavefront 15 onto a detector subaperture 80 a of thedetector array 30. The detector subaperture 80 a has a width dSH along the axis shown in FIG. 4. The lenslet 25 a has a nominal focal length of f and a nominal diameter that is substantially equal to the spacing between the lenslets of thelenslet array 20. The lenslet 25 a also has an optical axis 70 a defined by a line passing through the center of the lenslet 25 a and extending in a direction that is approximately normal to the center portion of the back surface 65 a of the lenslet 25 a. - The portion75 a of the
wavefront 15 enters the lenslet 25 a at an angle θ relative to a line 76 a that is substantially perpendicular to the optic axis 70 a (for purposes of this illustration, angular component of the portion 75 a along a line into the page of FIG. 4 is assumed to be zero). The portion 75 a is focused onto the detector subaperture 80 a to form a focus point 55 a located a distance Δd from the intersection of the optical axis 75 a with the detector subaperture 80 a. The angle θ may be approximately correlated to the distance Δd by the relationship: - θ=atan (Δd/f) (1)
- where Δd and f have the same dimensional units. When the angle θ is approximately zero, then Δd is also approximately zero and the focus point55 a is located at the intersection of the optical axis 70 a with the detector subaperture 80 a. When θ is positive, as shown in FIG. 4, Δd has a positive value that increases as θ increases. In a Shack-Hartmann wavefront sensor it is generally required that the distance Δd be less than one-half the detector subaperture width dSH, since a larger value of Δd would mean that the focus point 55 a was in the detector subaperture of an adjacent lenslet from the lenslet array, thus producing either an error or an ambiguity.
- FIG. 5a illustrates two possible problems that can be produced using a prior-art Shack-Hartmann wavefront sensor when the incident wavefront has portion in which the slope exceeds a predetermined limit. In the first instance, light from the
wavefront 15 is focused by thelenslets detector subaperture Equation 1 assumes, in this case incorrectly, that thefocus point 55 b is from light focused by thelenslet 25 b and visa versa. - In the second instance, light from the
wavefront 15 is focused by thelenslets detector subaperture 80 e. This situation creates two ambiguities. First, since there is no focus point inside thedetector subaperture 80 d, the local slope of the wavefront at thelenslet 25 d is indeterminate. Second, since there are two focus points (55 d and 55 e) inside thedetector subaperture 80 e, the local slope of the wavefront at the lenslet 25 e is also indeterminate, since it cannot be determined which of the focus points 55 d, 55 e should be used to calculate the local wavefront slope for the portion received by thelenslet 25 e. - Other problems of a similar nature may also be produced when the incident wavefront has portion in which the slope exceeds a predetermined limit. For instance, two focus points may completely or partially overlap one another, making it difficult or impossible to either detect or resolve two focus points. In the former case, only one focus point is detected and there are fewer focus points than there are lenslets. Also, a local wavefront slope into one or more of the
lenslets 25 may be so great that the some of the focus points may are disposed at locations that are even beyond any of the adjacent detector subapertures. - FIG. 5b illustrates one prior-art method of solving the problems illustrated in FIG. 5a. The prior-art solution is to replace the
lenslet array 20 with adifferent lenslet array 20′, wherein each of thelenslets 25′ has focal lengths of f′ that is less than f Using this approach, light fromlenslets 25 b′, 25 c′, and 25 d′ all remain within their correspondingdetector subapertures same detector array 30 having the same pixel resolution is used. - Principle of Operation
- FIG. 6 may be used to illustrate how the
mask 35 can increase the dynamic range of thewavefront sensor 10 as compared to a prior-art Shack-Hartmann wavefront sensor using a detector array equivalent to thedetector array 30 and lenslets equivalent to thelenslets 25. FIG. 6 shows alenslet 25 f that may be used to focus light form thewavefront 15 onto thedetector array 30. Twolenslets lenslet 25 f. Two more lenslets 25 j, 25 k are disposed adjacent to thelenslets lenslet 25 f. For a traditional Shack-Hartmann sensor not having themask 35, thedetector subapertures detector array 30 that may be used by the correspondinglenslets wavefront 15. - For this illustrative example, each
transmissive region 50 has a width that is substantially equal to the spacing s of thelenslets 25, and thetransmissive regions 50 are arranged such that everyother lenslet 25 from thearray 20 focuses light from thewavefront 15 onto thedetector array 30. Thus, when themask 35 is disposed to a first position 85 a, as shown in FIG. 6a, thelenslets detector array 30, while thelenslets lenslet 25 m are prevented from focusing light onto thedetector array 30. When themask 35 is disposed to a second position 85 b, as shown in FIG. 6b, thelenslets detector array 30, while thelenslets detector array 30. - When the
mask 35 at the first position 85 a, thelenslet 25 f is focuses light from thewavefront 15 onto thedetector array 30, while theadjacent lenslets detector array 30 by theopaque regions 45 of themask 35. Since theadjacent lenslets detector array 30, the portion of thedetector array 30 that is available to thelenslet 25 f for making wavefront slope measurements is aneffective detector subaperture 90 f, which is seen to be larger than thedetector subaperture 80 f. - The extent of the
effective detector subaperture 90 f along the face of thedetector array 30 is from the centers of theadjacent detector subapertures effective detector subaperture 90 f is limited in this way because theadjacent lenslets effective detector subaperture 90 f is approximately twice the size of thedetector subaperture 80 f (i.e., the subaperture oflenslet 25 f without the mask 35). Therefore, in this example, the dynamic range for thelenslet 25 f, in terms of the maximum wavefront slope that can be measured, is approximately twice that of an equivalent prior-art Shack-Hartmann wavefront sensor not using themask 35. In similar fashion, the dynamic range of theother lenslets 25 in thearray 20 corresponding to thetransmissive regions 50 of the mask 35 (e.g., thelenslets mask 35. - Continuing the illustrative example, FIG. 6b shows the
mask 35 at the second position 85 b. Thelenslets wavefront 15 onto thedetector array 30, now focus light form thewavefront 15 onto thedetector array 30, while theadjacent lenslets detector array 30. Since theadjacent lenslets lenslets mask 35. Thus, all thelenslets 25 of thearray 20 have a dynamic range that is approximately twice that of an equivalent prior-art Shack-Hartmann wavefront sensor does not use the mask 35 (i.e., half thelenslets 25 when themask 35 is located at the first position 85 a and the other half of thelenslets 25 when themask 35 is located at the second position 85 b). - Mask Step Method
- In certain embodiment, a method for measuring the
wavefront 15, herein referred to as the mask step method, comprises a first step of providing thewavefront sensor 10. The method further comprises a second step of disposing themask 35 to the first location 85 a wherein a first plurality of lenslets (e.g., lenslets 25 j, 25 f, 25 k in FIG. 6) from thearray 20 oflenslets 25 focus light from thewavefront 15 onto thedetector array 30. The method further comprises a third step of moving themask 35 to the second location 85 b, wherein a second plurality of lenslets 105 (e.g., lenslets 25 g, 25 h, 25 m in FIG. 6) from thearray 20 of lenslets focus light from thewavefront 15 onto thedetector array 30. - The use of six
lenslets 25 in FIG. 6 is for illustrative purposes only. Generally, the number oflenslets 25 in thearray 20 is larger than the six lenslets shown in FIG. 6, although the mask step method may be used when thearray 20 comprises as few as twolenslets 25. Using the mask step method, each of thelenslets 25 in thearray 20 is provided with an effective subaperture (e.g., theeffective detector subaperture 90 f) that is larger than the subaperture provided by an equivalent prior-art Shack-Hartmann sensor not having the mask 35 (e.g., thedetector subaperture 80 f). - In certain embodiments, the
detector subapertures 80 are in the form of a one-dimensional array and thepattern 40 of themask 35 is configured as in FIG. 6 wherein every other lenslet of thearray 20 focuses light from thewavefront 15 onto thedetector array 30. In such embodiments, the mask step method is used once to provide an increased dynamic range compared to a Shack-Hartmann type wavefront sensor that does not use this method. - In other embodiments, the
pattern 40 of themask 35 is configured wherein every nth lenslet of thearray 20 focuses light onto thedetector array 30. In such embodiments, the third step of the mask step method above may be repeated (n−2) times in order that each lenslets 25 in thearray 20 focuses light form thewavefront 15 sometime during the method. - In yet other embodiments, the
array 20 oflenslets 25 anddetector subapertures 80 are in the form of a two-dimensional arrays and the third step of the mask step method is repeated sufficient times so that eachlenslet 25 focuses light from thewavefront 15 at least once during the method. In such embodiments, thepattern 40 of themask 35 comprises a two-dimensional pattern 40. For example, themask 35 illustrated in FIG. 2 comprises the two-dimensional pattern 40 shown and may be used in conjunction with the 5×5array 20 oflenslets 25 shown in FIG. 3. - Two-dimensional Mask Step Method
- FIG. 7 may be used to illustrate one method of using the two-
dimensional pattern 40 of themask 35 shown in FIG. 2. Since FIG. 7 is a front view of thewavefront sensor 10, thewavefront 15 is not shown. Likewise, thedetector array 30 is not shown in FIG. 7 since it is located behind and, therefore, hidden by themask 35 and thearray 20 oflenslets 25. - Referring to FIG. 7, a preferred embodiment of the present invention comprises a method for measuring the
wavefront 15, wherein themask 35 comprises a two-dimensional pattern 40. The method, referred to herein as the two-dimensional mask step method, comprises a first step of providing thewavefront sensor 10. The method further comprises a second step of disposing themask 35 to a first location (e.g., that shown in FIG. 7a), wherein a first plurality oflenslets 110 from thearray 20 focuses light from thewavefront 15 onto thedetector array 30. The method further comprises a third step of moving themask 35 to a second location (e.g., that shown in FIG. 7b), wherein a second plurality oflenslets 115 from thearray 20 focuses light from thewavefront 15 onto thedetector array 30. The method further comprises a fourth step of moving themask 35 to a third location (e.g., that shown in FIG. 7c), wherein a third plurality oflenslets 120 from thearray 20 focuses light from thewavefront 15 onto thedetector array 30. The method further comprises a fifth step of moving themask 35 to a fourth location (e.g., that shown in FIG. 7d), wherein a fourth plurality of lenslets 125 from thearray 20 focuses light from thewavefront 15 onto thedetector array 30. - The two-dimensional mask step method utilizes a
mask 35 having a temporally fixedpattern 40 that is spatially varied by moving themask 35 to four different locations. During steps 2-5 of the method, themask 35 is moved such that eachtransparent region 50 defines a 2×2 sub-array oflenslets 25, wherein each lenslet 25 in the sub-array successively focus light from thewavefront 15 onto thedetector array 30. Using the method, each of thelenslets 25 in thearray 20 has a corresponding effective detector subaperture 90 that has approximately four times more area on thedetector array 30 than the correspondingdetector subaperture 80 provided by an equivalent prior-art Shack-Hartmann sensor not utilizing the two-dimensional mask step method. Thus, thewavefront sensor 10 is able to measure larger wavefront slopes without ambiguity than the equivalent Shack-Hartmann sensor that does not incorporate themask 35. - The two-dimensional mask step method, using the
pattern 40 shown in FIG. 2, may be used to remove ambiguities produced by prior-art Shack-Hartmann sensors occurring when local wavefront slopes cause light received by a lenslet to be focused onto the subaperture of an adjacent lenslet. Using thepattern 40 shown in FIG. 2, no ambiguity is produced so long as the focused light does not lie beyond the center of an adjacent subaperture corresponding to an adjacent lenslet. For example, if themask 35 shown in FIG. 6 represents one row or column of a two-dimensional pattern 40, the two-dimensional mask step method produces no ambiguity when thefocus point 55 f produced by thelenslet 25 f does not lie beyond thepoint 130 on the detector subaperture 80 g, wherein thepoint 130 represents the intersection ofdetector array 30 with the optical axis of the lenslet 25 g. - Modified Two-Dimensional Mask Step Method
- In certain embodiments, the temporally fixed
pattern 40 is configured such that the two-dimensional pattern 40 comprises a set of mtransmissive regions 50 configured such that the spacing between thetransmissive regions 50 along each of two orthogonal axes is everynth lenslet 25 of thearray 20. Using this pattern themask 35 may be moved in such a manner that eachtransparent region 50 defines an area that covers an n×n sub-array oflenslets 25, wherein each lenslet 25 in the n×n sub-array successively focus light from thewavefront 15 onto thedetector array 30. In certain embodiments, such apattern 40 is used in conjunction with modified version of the two-dimensional mask step method, referred to herein as the modified two-dimensional mask step. - The modified two-dimensional mask step method comprises a first step of providing the two-
dimensional pattern 40 on themask 35 having the set of mtransmissive regions 50 configured such that the spacing between thetransmissive regions 50 along each of two orthogonal axes is everynth lenslet 25 of thearray 20. The size of each transmissive region is preferably substantially equal to that of anindividual lenslet 25. The method comprises a second step of disposing themask 35 to the first location wherein a first plurality oflenslets 25 from thearray 20 focus light from thewavefront 15 onto thedetector array 30. The method further comprises a third step of moving themask 35 to (n2−1) different positions such that each of the m transmissiveregions 50 allows light from thewavefront 15 to be focused onto thedetector array 30 by eachlenslet 25 within an n×n sub-array oflenslets 25. - Using the modified two-dimensional mask step method, each of the
lenslets 25 in thearray 20 has a corresponding effective detector subaperture 90 that has approximately n2 times more area on thedetector array 30 than the correspondingdetector subaperture 80 provided by an equivalent prior-art Shack-Hartmann sensor not utilizing the two-dimensional mask step method. Thus, thewavefront sensor 10 is able to measure larger wavefront slopes without ambiguity than the equivalent Shack-Hartmann sensor that does not incorporate themask 35. - When using the either the two-dimensional mask step method or the modified two-dimensional mask step method, the
mask 35 may be located either in front of or behind thearray 20 oflenslets 25. Other methods utilizing different algorithms for moving themask 35 may alternatively be used in conjunction with the various embodiments of the temporally fixedpatterns 40 discussed above herein. Also, different embodiments of the temporally fixedpatterns 40 may be used to increase the dynamic range of thedevice 10 over prior-art Shack-Hartmann wavefront sensors not utilizing themask 35. - In certain embodiments, the
mask 35 comprises an SLM and the two-dimensional, temporally fixedpattern 40 is produced by addressing the pixels of the SLM in a predetermined manner using an appropriate electronic input into the SLM. In such embodiments, thepattern 40 is spatially varied by varying the electronic input into the SLM in a predetermined manner such that thepattern 40 is moved spatially, but is unchanged in terms of the overall shape and size of the pattern and the components thereof. - Point Ambiguity Elimination Method
- FIG. 8 may be used to describe another embodiment of the present invention, wherein a method for measuring the
wavefront 15 comprises a first step of providing thewavefront sensor 15 and disposing thearray 20 oflenslets 25 such that two oflenslets wavefront 15 onto a point P on thedetector array 30. The method additionally comprises a second step of disposing themask 35 such that only one of the twolenslets 25 focuses light from thewavefront 15 onto the point P. - As illustrated in FIG. 8, the
wavefront 15 is disposed such that thelenslets detector array 30. In FIG. 8a themask 35 is positioned so that only light from thewavefront 15 entering thelenslet 25 n is focused onto the point P. The dotted line fromlenslet 25 p indicates light from thewavefront 15 that would be focused to the point P on thedetector array 30 if themask 35 were removed or moved to another position such as that shown in FIG. 8b. In FIG. 8b themask 35 is positioned so that only light from thewavefront 15 entering thelenslet 25 p is focused onto the point P. The dotted line fromlenslet 25 n indicates light from thewavefront 15 that would be focused to the point P on thedetector array 30 if themask 35 were removed or moved to another position such as the position shown in FIG. 8a. - Using the two different positions of the
mask 35, it can be determined that the light contained in the point P is produced by light from thewavefront 15 that is focused by both thelenslet 25 n and thelenslet 25 p. Therefore, the signal produced by focused light at the point P on thedetector array 30 may be used to determine the average slope of thewavefront 15 within the areas corresponding to thelenslets - Single Aperture Method
- In certain other embodiments, such as that shown in FIG. 9, the
array 20 oflenslets 25 is replaced by asingle lens 170 and thewavefront sensor 10 contains amask 35 that comprises anaperture 175 adapted to transmit from light from thewavefront 15. Thelens 170 preferably has a diameter that is at least equivalent to the largest dimension of the array detector 30 (e.g., the diagonal length of a rectangular or square array detector). Thelens 170 may be a refractive element comprising a single material or a achromatic lens comprising two or more materials. Alternatively, thelens 170 may any suitable imaging optical element such as a compound lens, curved mirror, holographic optical element, or diffractive optical element. - The
aperture 175 is typically circular or square with a diameter that is sufficiently small so that light from only a small portion of thewavefront 15 is received bylens 170. The diameter of theaperture 175 is preferably less than about 3 millimeter, more preferably less than about 1 millimeter, and even more preferably less than about 500 micrometers. - The
wavefront sensor 10 schematically illustrated in FIG. 9 may be used in a method for measuring a wavefront comprising a first step of providing awavefront sensor 10 that comprises thedetector array 30, thelens 170, and themask 35 having theaperture 175. The method additionally comprises a second step of disposing themask 35 to a first location, wherein light from a first portion of thewavefront 15 is transmitted by theaperture 175 and is focused by thelens 170 onto thedetector array 30 to produce a first signal. The method further comprises a third step of moving themask 35 to a second location, wherein light from a second portion of thewavefront 15 is transmitted by theaperture 175 and is focused by thelens 170 onto thedetector array 30 to produce a second signal. The method also comprises a fourth step of using the first signal to determine the slope of the first portion of thewavefront 15 and using the second signal to determine the slope of the second portion of thewavefront 15. - SLM Methods
- In certain embodiments, such as that shown in FIG. 10, the
array 20 oflenslets 25 is incorporated into themask 20. In such embodiments, thewavefront sensor 10 comprises anSLM 180 having a first plurality ofzones 185 and a second plurality ofzones 190. The first plurality ofzones 185 is adapted to substantially block light from a first portion of the wavefront 15 (not shown) such that light from the first portion of thewavefront 15 is not received by the detector array. The second plurality ofzones 190 is adapted to form a plurality of focusingelements 195 that focus light form thewavefront 15 to produce a corresponding plurality of foci on thedetector array 30. The plurality of foci produces a plurality of signals that may be used for estimating the slope at a plurality locations on thewavefront 15 corresponding to the locations of the plurality of focusing elements 165. TheSLM 180 may be alternatively used in any of the previous embodiments of thewavefront sensor 10 disclosed above herein to replace themask 35 and thearray 20 oflenslets 25. TheSLM 180 may also be for any of the methods discussed above herein utilizing thewavefront sensor 10. - It is to be understood that the patent rights arising hereunder are not to be limited to the specific embodiments or methods described in this specification or illustrated in the drawings, but extend to other arrangements, technology, and methods, now existing or hereinafter arising, which are suitable or sufficient for achieving the purposes and advantages hereof.
Claims (47)
1. A device for measuring a wavefront, the device comprising:
a detector array configured to detect light passing through an array of lenslets; and
a mask having a fixed pattern comprising an opaque region that is substantially opaque to light from the wavefront and a transmissive region that is transmissive of light from the wavefront;
wherein the mask and the array of lenslets are disposed such that light from the wavefront that is transmitted by the transmissive region is focused onto the detector array by the array of lenslets; and
wherein the mask is adapted to be selectably disposed to any one of a plurality of predetermined positions, wherein a subset of lenslets from the array of lenslets focuses light from the wavefront onto the detector array, depending on which of the plurality of predetermined positions is selected.
2. The device of claim 1 , wherein the opaque region is totally opaque to light from the wavefront.
3. The device of claim 1 , wherein the array of lenslets is disposed in a two-dimensional grid that samples at least a portion of the wavefront.
4. The device of claim 1 , wherein the array of lenslets has a fill factor of one or less.
5. The device of claim 4 , wherein the lenslets forming the array of lenslets are spaced substantially equally from one another.
6. The device of claim 4 , wherein the lenslets forming the array of lenslets are spaced apart unequally relative to one another.
7. The device of claim 1 , wherein the mask comprises two or more transmissive regions that are transmissive of light from the wavefront.
8. The device of claim 7 , wherein the fixed pattern is configured such that the spacing between the transmissive regions along each of two orthogonal axes is every nth lenslet of the array, where n is greater than or equal to two.
9. The device of claim 8 , where n is equal to two.
10. The device of claim 7 , wherein the fixed pattern is configured such that the spacing between the transmissive regions is every nth lenslet of the array, where n is greater than or equal to two.
11. The device of claim 10 , where n is equal to two.
12. The device of claim 1 , wherein the device is configured to measure a wavefront originating from a human eye.
13. The device of claim 1 , wherein the detector array is a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) detector array.
14. The device of claim 1 , wherein the detector array further comprises a plurality of detector subapertures, each detector subaperture corresponding to a lenslet in the array.
15. The device of claim 1 , wherein the transmissive region transmits light from the wavefront to a region of the detector array that is not transverse relative to the transmissive region.
16. The device of claim 1 , wherein the locations of a plurality of focus points of the lenslets is correlated to the nominal slope of the wavefront over the aperture of each lenslet focusing light from the wavefront.
17. The device of claim 1 , wherein the transmissive regions have substantially the same shape as the front face of the lenslets.
18. The device of claim 1 , wherein the transmissive regions have substantially the same area as the front face of the lenslets.
19. The device of claim 1 , wherein the array of lenslets is positioned between the mask and the detector array.
20. The device of claim 1 , wherein the mask is positioned between the array of lenslets and the detector array.
21. The device of claim 20 , wherein the mask is positioned in a conjugate plane with a pupil.
22. The device of claim 21 , wherein the conjugate plane is generated using an image relay system.
23. The device of claim 1 , wherein the mask is configured to provide a dynamic range of at least double the dynamic range the device would be capable of without a mask.
24. A device for measuring a wavefront, the device comprising:
a detector array configured to detect light passing through a lens; and
a mask having a fixed pattern, the mask comprising an opaque region that is substantially opaque to light from the wavefront and a transmissive region that is transmissive of light from the wavefront;
wherein the mask and the lens are disposed such that light from the wavefront that is transmitted by the transmissive region is focused onto the detector array by the lens; and
wherein the mask is adapted to be selectably disposed to any one of a plurality of predetermined positions, wherein the lens focuses light from the wavefront onto the detector array, depending on which of the plurality of predetermined positions is selected.
25. The device of claim 24 , wherein the opaque region is totally opaque to light from the wavefront.
26. The device of claim 24 , wherein the mask comprises two or more transmissive regions that are transmissive of light from the wavefront.
27. The device of claim 26 , wherein the fixed pattern is configured such that the spacing between the transmissive regions along each of two orthogonal axes is every nth lenslet of the array, where n is greater than or equal to two.
28. The device of claim 27 , where n is equal to two.
29. The device of claim 24 , wherein the device is configured to measure a wavefront originating from a human eye.
30. The device of claim 24 , wherein the detector array is a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) detector array.
31. The device of claim 24 , wherein the lens is positioned between the mask and the detector array.
32. The device of claim 24 , wherein the mask is positioned between the lens and the detector array.
33. The device of claim 24 , wherein the mask is configured to provide a dynamic range of at least double the dynamic range the device would be capable of without a mask.
34. A method for measuring a wavefront comprising:
providing a wavefront sensor containing a detector array, an array of lenslets, and a mask having a fixed pattern containing one or more opaque regions that are substantially opaque to light from the wavefront and one or more transmissive regions that are transmissive of light from the wavefront;
disposing the array of lenslets such that at least two lenslets from the array of lenslets are configured to focus light from the wavefront onto the detector array; and
moving the mask such that only one of the at least two lenslets focuses light from the wavefront onto the detector array.
35. A method for measuring a wavefront comprising:
providing a wavefront sensor containing a detector array, an array of lenslets, and a mask having a fixed pattern containing one or more opaque regions that are substantially opaque to light from the wavefront and one or more transmissive regions that are transmissive of light from the wavefront;
disposing the mask to a first location such that a first plurality of lenslets from the array of lenslets focuses light from the wavefront onto the detector array; and
moving the mask to a second location such that a second plurality of lenslets from the array of lenslets focus light from the wavefront onto the detector array.
36. The method of claim 35 , further comprising moving the mask to a third location such that a third plurality of lenslets from the array of lenslets focuses light from the wavefront onto the detector array.
37. The method of claim 36 , further comprising moving the mask to a fourth location such that a fourth plurality of lenslets from the array of lenslets focuses light from the wavefront onto the detector array.
38. A method for measuring a wavefront comprising:
providing a wavefront sensor containing a detector array, a lens, and a mask having an aperture adapted to transmit light from the wavefront;
disposing the mask to a first location, wherein light from a first portion of the wavefront is transmitted through the aperture and is focused by the lens onto the detector array to produce a first signal;
moving the mask to a second location, wherein light from a second portion of the wavefront is transmitted through the aperture and is focused by the lens onto the detector array to produce a second signal.
39. The method of claim 38 , further comprising processing the first and second signals with a computer to determine the wavefront measurement.
40. The method of claim 38 , further comprising:
using the first signal to determine the slope of the first portion of the wavefront; and
using the second signal to determine the slope of the second portion of the wavefront.
41. The method of claim 38 , further comprising moving the mask to a third location, wherein a third portion of the wavefront is transmitted through the aperture and is focused by the lens onto the detector array to produce a third signal.
42. The method of claim 41 , further comprising using the third signal to determine the slope of the third portion of the wavefront.
43. The method of claim 41 , further comprising moving the mask to a fourth location, wherein a fourth portion of the wavefront is transmitted through the aperture and is focused by the lens onto the detector array to produce a fourth signal.
44. The method of claim 43 , further comprising using the fourth signal to determine the slope of the fourth portion of the wavefront.
45. A method for measuring a wavefront comprising:
providing a wavefront sensor containing a detector array, an array of lenslets, and a mask having a fixed pattern containing one or more opaque regions that are substantially opaque to light from the wavefront and one or more transmissive regions that are transmissive of light from the wavefront, wherein the spacing between the transmissive regions along each of two orthogonal axes is every nth lenslet of the array of lenslets, where n is greater than or equal to two;
disposing the mask to a first position such that a first plurality of lenslets from the array of lenslets focuses light from the wavefront onto the detector array;
disposing the mask to a plurality of different positions such that each of the transmissive regions allows light to be focused from the wavefront onto the detector array.
46. The method of claim 45 , wherein the step of disposing the mask to a plurality of different positions comprises disposing the mask to (n2−1) different positions.
47. The method of claim 45 , where n is equal to two.
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